1. Field of the Invention
The invention relates to a process for reducing the heat build-up on dynamic stressing (energy dissipation) in vulcanised rubber products.
2. Description of the Related Art
It is known that phenol novolaks or modified phenol novolaks cured with hexamethylene tetramine (HMT) or certain melamine formaldehyde resins like the highly etherified compound hexamethoxymethyl melamine (HMMM) give good reinforcement in the vulcanisate. In particular, there is a marked increase in vulcanisate hardness and tensile strength (stress), in each case measured at a specified extension. For processing, the melamine resin derived curing agents (in particular HMMM) are used in liquid form or, as is preferred in the rubber industry, adsorbed on solid carrier materials (e.g. finely divided silica or diatomaceous earth) to become free-flowing powders with a mass fraction of from about 65 to 75% of active ingredient (in this case HMMM) (see, for example xe2x80x9cPhenolic Resins for Rubber Reinforcementxe2x80x9d; E. Leicht and R. Sattelmeyer; Kautschuk+Gummi Kunststoffe, Vol. 40, No. 2/87; page 126).
According to the prior art, however, reinforcing resins in the vulcanisate do not merely increase hardness and tensile strength, but also undesirably affect the viscoelastic performance of the vulcanisates by increasing the amount of mechanical energy converted into heat and dissipated within the rubber products. This property of heat evolution and the resultant temperature increase in the event of dynamic load (heat build-up) is significant for those rubber products which in service are exposed to severe dynamic loads, for example parts in automotive tires. Now, according to the prior art, this undesirable heat evolution becomes more pronounced in a given rubber composition as the resin becomes more effective in its reinforcing action.
EP-A 0 013 330 discloses that rubber mixtures can be vulcanised in the presence of novolak resins and of reactive melamine resins obtainable by reacting melamine with from 0.5 to 6 mol of aldehyde per mole of melamine. The methylol groups present in the melamine resins may be free or may be etherified. No study was made of the effect which the melamine-resin curing agent used has on heat evolution in dynamically stressed rubber products.
There is increasing demand by the industry for systems of reinforcing resins and curing agents therefor for use in rubber products which combine good reinforcing action with moderate heat build-up in the event of dynamic load.
Surprisingly, it has now been found that the use of a combination of novolaks with selected non-etherified or at least partially etherified reaction products of formaldehyde and triazines gives good vulcanisate reinforcement together with a heat build-up markedly lower than that obtained using the same reinforcing resin and using a prior-art highly etherified melamine resin, such as HMMM (hexamethoxymethyl melamine) or using another formaldehyde source as curing agent. It was not to be expected that this combination of reinforcing resins and curing agents can be used for the purpose of reducing heat evolution in dynamically stressed rubber products.
The invention therefore provides a process for reducing the heat build-up on dynamic stressing of vulcanised rubber products, which comprises admixing for this purpose, to vulcanisable rubber mixtures, as a combination of reinforcing resin and curing agent, mixtures made from non-selfcuring novolaks A as reinforcing resins and non-etherified or partially etherified triazine resins B as curing agents for these reinforcing resins. The triazine resins suitable for the invention are those with a molar ratio of formaldehyde to triazine (F/T) of from 0.5 to 5.5 mol/mol, preferably 0.75 to 5.25 mol/mol, and especially preferred 1.0 to 5.0 mol/mol, and which are non-etherified or at least partially etherified, with a degree of etherification of preferably from 0 to 80%, based on bonded formaldehyde. The alcohols used for etherification include linear and branched aliphatic alcohols having from 1 to 18 carbon atoms.
The invention also provides combinations of novolaks A and partially etherified triazine resins B as reinforcing resins in rubber mixtures and curing agents for these, the triazine resins being condensation products of formaldehyde and of triazines selected from melamine, acetoguanamine, benzoguanamine and caprinoguanamine, or else mixtures of these, preferably only melamine, with the ratio F/T of the amount of substance of formaldehyde to the amount of substance of triazine in the resins B of from 0.5 to 5.5 mol/mol, preferably 0.75 to 5.25 mol/mol, and especially preferred 1.0 to 5.0 mol/mol, and having an amount of substance of alkoxy groups per 1 mol of triazine of (F/T-1.75 mol/mol), but at least 0.5 mol/mol, preferably at least 0.75 mol/mol, and especially preferred at least 1 mol/mol, and having an amount of substance of alkoxy groups per 1 mol of triazine of not more than F/T, preferably not more than 90% of F/T, and especially preferred not more than 80% of F/T.
The invention also provides a method of use of these combinations of novolaks A and partially etherified triazine resins B for reducing heat build-up in dynamically stressed vulcanised rubber mixtures comprising admixing these combinations to the rubber formulation.
The process for reduction of the heat build-up according to the invention can be used in a very wide variety of vulcanisable rubber mixtures to give rubber products with low heat evolution. Suitable types of rubber which may be used are sulfur-vulcanisable rubbers, e.g. natural rubber NR), styrene-butadiene rubber (SBR), polyisoprene rubber (IR), polybutadiene rubber (BR), nitrile rubber (NBR), butyl rubber (IIR), ethylene-propylene rubber (EPM), ethylene-propylene-diene rubber (EPDM) and mixtures of these, as conventionally used in the tire industry or for industrial rubber products.
Examples of usual other additives are fillers, such as carbon black, silica, chalk, and kaolin. Other constituents which may be added to the mixtures are vulcanizing agents, accelerators, activators, processing aids, antioxidants and plasticisers, e.g. mineral oils.
Suitable reinforcing resins A are non-selfcuring phenolic resins, such as phenol novolaks, or polynuclear phenols. Examples of particularly suitable novolak resins are those made from phenol and/or from polyfunctional mono- or polynuclear phenols, such as resorcinol, or from bisphenols and/or in particular from mono- or polyhydric phenols substituted with a hydrocarbon radical, for example alkyl- or aralkylphenols whose substituent has from 1 to 20 carbon atoms, preferably from 4 to 18 carbon atoms, for example tert.-butylphenol, octylphenol, the phenols cardol and cardanol present as a mixture in the oil from cashew nut shells, and also those from styrenated phenols, or fatty-acid-modified, e.g. linseed-oil-fatty acid-modified, phenol or phenylphenol. Useful phenolic resins may also comprise mixtures of the individual resins and resins made from mixtures of these phenols.
These reinforcing resins are prepared by reacting the phenol or the mixture of phenols with aldehydes, such as acetaldehyde, but preferably formaldehyde, in an acid medium as in the prior art. The novolaks may also have some content of plasticizing materials, such as polyethylene glycols, tall oil, or other conventional plasticisers. The phenolic resins may also have been modified with natural resins, such as colophony or tall resin, or with terpenes, and in specific cases the natural resin content may even predominate.
Preferred are those novolaks A where the phenols comprise a mass fraction of at least 10% of alkylphenols having at least one alkyl group per phenol ring and having an alkyl radical of from 1 to 20 carbon atoms. Especially preferred are reaction products of mixtures of phenol and tert-butylphenol with formaldehyde as novolaks A.
The triazine resins B used according to the invention are non-etherified or at least partially etherified reaction products of aminotriazines with aldehydes, preferably with formaldehyde. Besides melamine, also guanamines, e.g. acetoguanamine, benzoguanamine and caprinoguanamine may be used as aminotriazines, or triazines. The preferred triazine is melamine or a mixture of melamine with one of the other triazines mentioned, the mass fraction of these triazines in the triazine mixture being not more than 15%, preferably not more than 10%.
The molar ratio of alkoxy groups to triazine molecules (A/T) is related to the degree of etherification which, in the case of formaldehyde resins, is the ratio between the amount of substance of etherified methylol groups (=alkoxymethyl groups) and the total of the amount of substance of of etherified (alkyoxymethyl) and unetherified (hydroxymethyl) methylol groups in the relevant triazine resin and can be expressed as the product of F/T and the degree of etherification. The etherification agents used may comprise linear or branched aliphatic alcohols having from 1 to 18 carbon atoms, in particular from 1 to 4 carbon atoms, in particular methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, sec.-butanol or tert.-butanol. It is preferable for the resins B to have been etherified with methanol or isobutanol, particularly preferably with methanol. It is further preferable that the triazine used to prepare the resin B together with formaldehyde is melamine or a mixture of melamine with other triazines, the mass fraction of melamine in the triazine mixture being at least 50%, preferably at least 65%, in particular at least 85%.
Particularly suitable are those resins B where only melamine is used as triazine, having an amount of substance of from 0.5 to 5.5 mol, preferably from 1.0 to 5.0 mol, in particular from 1.5 to 4.5 mol, of bonded formaldehyde (in the form of N-methylol groups) per 1 mol of melamine, and having an amount of substance of alkoxy groups per 1 mol of triazine of (F/T-1.75 mol/mol), but at least 0.5 mol/mol, preferably at least 0.75 mol/mol, and especially preferred at least 1 mol/mol, and having an amount of substance of alkoxy groups per 1 mol of triazine of not more than F/T, preferably not more than 90% of F/T, and especially preferred not more than 80% of F/T.
The aqueous resin solutions comprising the partially etherified triazine resin can be converted to powder form by spray drying [Kunststoffhandbuch [Plastics Handbook] Vol. 10, Thermosets, ed. W. Woebcken, Hanser-Verlag, 2nd edn. 1988, p. 48; Ullmann""s Encyclopedia of Industrial Chemistry, Vol. A2, ed. W. Gerhartz, 5th edn. 1985, p. 116-141]. The resin solutions may also be absorbed onto finely divided silica powders as carriers, and then dried.
Preferably, the ratio of the mass of the novolak A to the mass of the triazine resin B is from 1:2 to 5:1.
The vulcanisable rubber mixtures of the invention are prepared by known methods. For example, they are prepared in internal mixers or on mixing rolls. To improve distribution of the reinforcing resins A, it is useful to increase the temperature of the mixture during the mixing procedure to above the melting range of the reinforcing resin A. However, to avoid premature reaction with the crosslinkable reinforcing resin A, care has to be taken that the curing agent B is not added before the second mixing stage, together with the sulfur and optionally, with the accelerators, and at temperatures which are not too high (preferably from 70 to 120xc2x0 C.).
The remaining constituents, i.e. the rubber or rubber mixture itself and all of the additives with the exception of triazine resin B, and with the exception of the accelerators, if any, and sulfur, are mixed in the first mixing stage in the usual manner, preferably at temperatures of from 100 to 170xc2x0 C., in internal mixers or on conventional heatable mixing rolls.
Examples of the use of the vulcanisates of the invention are use as industrial rubber products, such as dampers, rubber sleeves, bellows, conveyor belts, hose and V-belts, and in particular use in tires for vehicles.
The mixtures of the invention may also be used in layers which have to have good adhesion to reinforcing inserts, for example to fabrics or to braids. To improve adhesion, use may also be made of adhesion promoters as further additives.
In the vulcanisable rubber mixtures, the mass fraction of the mixtures made from reinforcing resin A and from triazine resins B may preferably be from 1 to 20%, in particular from 2 to 15%, particularly preferably from 2.5 to 10%, based on the total mass of the vulcanisable rubber mixture.
The examples below further illustrate the invention. For the purposes of the present invention, xe2x80x9csilicaxe2x80x9d in the examples is finely divided silicon dioxide.